The demand for reduced fuel consumption, together with environmental demands concerning chemical composition of exhaust gases increase the requirement to monitor the operation of combustion engines in very close detail. Misfiring influences exhaust gas chemical composition, fuel consumption, and can also negatively influence the working life of a combustion engine. With the help of continuous measurement misfiring can be detected and action be taken to ensure proper functioning is regained
With the control of large combustion engines such as for example ships engines, there is a requirement for accurate measurement of the torque in an output shaft. A combination of such a measurement and a measurement of combustion conditions in the cylinders, makes it possible to monitor and control the engine to get higher speeds and lower fuel consumption. Because of the growing demands for reduced consumption of fuel and continually increasing environmental demands on the chemical composition of exhaust gases the requirement to monitor the operation of combustion engines has increased. Misfiring influences exhaust gas chemical composition and can also negatively influence the working life of a combustion engine. With the help of continuous measurement misfiring and other factors affecting engine performance can be detected and action be taken to ensure proper functioning is regained.
However measurement of output shaft torque has to be extremely accurate and quick. The torque must be measured with a repeatable accuracy of fractions of a degree and with a time precision in the area of micro seconds.
Torque measurement may be based on the torque moment in an output shaft. The degree of torque is shown by the twisting along the length of the shaft and depends on measuring the difference between the angular inclination at one end of the shaft compared to the other end.
A known method to measure the angular position a rotating shaft is to mount a angle sensor of a known type on one end of the shaft. Such a method may only be applied when there is a length of shaft available where angular measurement is desired, rather than a shaft that is built-in or enclosed an impractical to reach. Often an output shaft from a machine is long and twists along its length. If one then wishes to measure the position of the shaft at a certain point, the twist up to the end of the shaft may be so great that it introduces serious errors in the measurement if the position of the end is measured. Measurement at a motor end rather than at a drive end of, for example, a long transmission shaft is then to be preferred.
A known method to measure a rotating shaft, which is independent of shaft length, is to apply a regular and legible pattern to the shaft in the place where a measurement is desired. There can for example be regular black stripes on a white background which may be read with an optical technique. Such devices have shown themselves to function well in clean environments but have difficulties in dirtier surroundings. Furthermore, it has been difficult to attach such devices in a satisfactory way to large machines with powerful vibrations and great mechanical forces.
A gearwheel is often mounted in just a position where a measurement is desired. In those cases devices have come into use that have been based on inductive techniques which measure the position of a one or more gear teeth, and through that position, obtain the angular inclination of the shaft. Commonly a large change in magnetic field is detected, which happens when the gear tooth enters the area of the magnetic field, which is to say, that it is the flank of the gear tooth which is detected.
It has been difficult to achieve better than relatively low accuracy with these devices even though the desired accuracy required for regulation of the adjacent apparatus is high. Such devices have not come into as widespread use as has been desired. The low accuracy of these devices depends on that the signal they give is not only dependent on the position of the gear tooth, but also of the appearance of the gear tooth, the distance between gear tooth and measuring device and the magnetic characteristics of the gear tooth. All these parameters may vary from tooth to tooth and with time, depending on that the gear teeth are subjected to wear and deformation while at the same time the gear wheel is seldom perfectly circular.
The inductive devices that have come into use for measurement as described above are based on a sinusoidal variations of a magnetic field. Inductive measurement based on use of a measurement supply current that is suddenly cut off have been described in U.S. Pat. No. 5,059,902 and shown a measurement technique that is more robust and facilitates separation of different characteristics of an object being measured. A difficulty with this measurement device is to determine position of an object being measured when distance to or shape of the object is changed.
In U.S. Pat. No. 5,270,646 a way is shown of how coils may be arranged so that measurement of an edge of a sheet can be carried out. In the same way the edge position of a gear tooth could be measured but a problem with that device is that extensive mathematical calculations are demanded, which leads to a problem with time when used with a rapidly rotating gearwheel. Moreover the calculations demand fast computing capacity, which could make such a device unnecessarily costly in order to measure the position of a gear tooth.
In SE 95035838 a way is shown to arrange coils in order to measure the position of an object. In the case of measurement of gear tooth position with a coil above the gear tooth the device will measure distance to the object but not a position of the object.